The present invention relates to ferromagnetic thin film memories and more particularly to a process for the fabrication of a thin film magnetoresistive memory device.
The process for manufacturing the magnetoresistive bit lines involves depositing one or more layers of magnetic material over an underlying layer or surface, and etching the deposited material to a desired configuration.
This process is described in U.S. Pat. No. 5,496,759 which is assigned to Honeywell Inc.
U.S. Pat. No. 4,731,757 dated Mar. 15, 1988, U.S. Pat. No. 4,780,848 dated Oct. 25, 1988, and U.S. Pat. No. 5,060,193 dated Oct. 22, 1991 which are assigned to Honeywell Inc. include background material on magnetoresistive memories and are hereby incorporated by reference.
U.S. Pat. No. 5,496,759, assigned to Honeywell Inc., includes details on the forming of magnetic memory bits and is hereby incorporated by reference. The present invention is an improvement to the process of U.S. Pat. No. 5,496,759.
The process of U.S. Pat. No. 5,496,759 provides first and second layers of thin film ferromagnetic materials separated by a layer of non ferromagnetic material and formed into an elongated shape having tapered ends to provide a magnetic bit. In use, information is kept as a binary bit having one of two alternative logic levels stored in the two layers or films by having the magnetization point in one direction or the other (but opposite in each of these layers to the direction in the other), generally along the easy axis of magnetization. If the direction of magnetization is caused to rotate from such a direction by external magnetic fields, the electrical resistance of the layers will change with this magnetization direction rotation because of the magnetoresistive properties of the films. Measurement of the resistance allows the state of the memory to be determined.
The process described in U.S. Pat. No. 5,496,759 generally provides reliable magnetic bits. However it has been observed that when this process is used for manufacturing magnetic memory bits, that some number of bits will fail after a number of repeated memory storage and retrieval operations, or read/write cycles. The failure mechanism is believed to be bit edge reversal which is sometimes called edge spin reversal. The elongated shape of the bit structure causes large demagnetization fields along the edge of the layers. In such large demagnetization fields, electron spins at the edge of the films are constrained to lie nearly parallel to these edges and the direction of elongation of these films. During read or write operations it is intended that bit edge reversal does not occur. Thus, a need exists for a magnetoresistive memory bit which allows an unlimited number of read/write cycles without bit edge reversal.